21.17 Tooltips

Tooltips are small windows that display text information at
the current mouse position. They activate when there is a pause in
mouse movement over some significant piece of text in a window, or the
mode line, or some other part of the Emacs frame such as a tool bar
button or menu item.

You can toggle the use of tooltips with the command M-x
tooltip-mode. When Tooltip mode is disabled, the help text is
displayed in the echo area instead. To control the use of tooltips at
startup, customize the variable tooltip-mode.

The variables tooltip-delay specifies how long Emacs should
wait before displaying a tooltip. For additional customization
options for displaying tooltips, use M-x customize-group
<RET> tooltip <RET>.

If Emacs is built with GTK+ support, it displays tooltips via GTK+,
using the default appearance of GTK+ tooltips. To disable this,
change the variable x-gtk-use-system-tooltips to nil.
If you do this, or if Emacs is built without GTK+ support, most
attributes of the tooltip text are specified by the tooltip
face, and by X resources (see X Resources).

GUD tooltips are special tooltips that show the values of
variables when debugging a program with GUD. See Debugger Operation.